Generally, the process for manufacturing integrated circuits on a silicon wafer substrate typically involves deposition of a thin dielectric or conductive film on the wafer using oxidation or any of a variety of chemical vapor deposition processes; formation of a circuit pattern on a layer of photoresist material by photolithography; placing a photoresist mask layer corresponding to the circuit pattern on the wafer; etching of the circuit pattern in the conductive layer on the wafer; and stripping of the photoresist mask layer from the wafer. Each of these steps, particularly the photoresist stripping step, provides abundant opportunity for organic, metal and other potential circuit-contaminating particles to accumulate on the wafer surface.
In the semiconductor fabrication industry, minimization of particle contamination on semiconductor wafers increases in importance as the integrated circuit devices on the wafers decrease in size. With the reduced size of the devices, a contaminant having a particular size occupies a relatively larger percentage of the available space for circuit elements on the wafer as compared to wafers containing the larger devices of the past. Moreover, the presence of particles in the integrated circuits compromises the functional integrity of the devices in the finished electronic product. Currently, mini-environment based IC manufacturing facilities are equipped to control airborne particles much smaller than 1.0 μm, as surface contamination continues to be of high priority to semiconductor manufacturers. To achieve an ultraclean wafer surface, particles must be removed from the wafer, and particle-removing methods are therefore of utmost importance in the fabrication of semiconductors.
The most common system for cleaning semiconductor wafers during wafer processing includes a series of tanks which contain the necessary cleaning solutions and are positioned in a “wet bench” in a clean room. Batches of wafers are moved in sequence through the tanks, typically by operation of a computer-controlled automated apparatus. Currently, semiconductor manufacturers use wet cleaning processes which may use cleaning agents such as deionized water and/or surfactants. Other wafer-cleaning processes utilize solvents, dry cleaning using high-velocity gas jets, and a megasonic cleaning process, in which very high-frequency sound waves are used to dislodge particles from the wafer surface. Cleaning systems which use deionized (DI) water currently are widely used in the industry because the systems are effective in removing particles from the wafers and are relatively cost-efficient. Approximately 4.5 tons of water are used for the production of each 200-mm, 16-Mbit, DRAM wafer.
Conventionally, wafers cleaned using DI water are subsequently dried using spin dryers. In a spin drying device, a cleaned wafer is rotated at high speeds in order to remove water remaining on the wafer after a rinsing step, using centrifugal force and air flow. The spin drying device is capable of drying wafers at a high throughput. Conventional spin drying devices are broadly classified into three types: multi-cassette dryers, single-cassette dryers and single-wafer dryers.
A typical conventional wet bench spin dryer, such as a KAIJO (trademark) spin dryer for drying residual DI water from semiconductor wafers, is generally indicated by reference numeral 10 in FIG. 1. The spin dryer 10 includes a chamber 11 which contains a pair of cassette cradles 12, each of which receives a wafer cassette 20, containing multiple wafers 21, from a robot arm 19 of a wafer transfer robot 18. A hinge 15 pivotally mounts a lid 13 on the chamber 11. The lid 13 is raised and lowered at the hinge 15 by actuation of a lid cylinder 14. A plastic stabilization chain 16 functions to stabilize the lid 13 in an even plane as the lid 13 is opened.
In operation, the wafer cassettes 20 are initially loaded onto the robot arm 19 of the wafer transfer robot 18, which raises the wafer cassettes 20, as shown by the dashed lines, preparatory to positioning the wafer cassettes 20 for subsequent loading in the cassette cradles 12 of the chamber 11, as shown in the solid lines. Simultaneously, the lid 13 is raised to expose the cassette cradles 12. Normally, the lid 13 remains open, and the robot arm 19 lowers the wafer cassettes 20 unimpeded into the respective cassette cradles 12. The lid 13 is then closed and the cassette cradles 12 are rotated in the chamber 11 to remove residual rinsing water from the wafers 21.
As further shown in FIG. 1, one of the problems commonly encountered in operation of the spin dryer 10 is that the welding joints of the hinge 15 become brittle and weaken after repeated use. Consequently, the typically heavy stainless steel lid 13 falls, as shown by the dashed lines, or the lid 13 shifts away from the normal position, hitting the robot arm 19 and damaging the semiconductor wafers 21 in one or both of the wafer cassettes 20. Another problem that frequently occurs is that the plastic stabilization chain 16 becomes distorted and fails to maintain the lid 13 in an even plane upon opening, thereby causing the robot arm 19 to strike the lid 13 during the cassette-loading operation. It has been found that over a three-year period, about 50 wafers were broken in the heretofore-described manner during the wafer-loading process for one spin dryer. Accordingly, a system is needed for ascertaining the position of a lid on a spin dryer and communicating this information to a wafer transfer robot to prevent the robot from transferring the wafers to the wafer-loading position in the event that the hinge or stabilization chain fails and the lid does not open properly.
An object of the present invention is to provide a system for preventing collision of a spin dryer lid with a wafer transfer robot as the robot loads wafers into a spin dryer.
Another object of the present invention is to provide a system for automatically halting transfer of wafer cassettes to a spin dryer in the event that the lid of the spin dryer fails to open properly.
Still another object of the present invention is to provide a dryer lid/robot collision prevention system which prevents wafer damage and scrapping.
Yet another object of the present invention is to provide a dryer lid/robot collision prevention system which ascertains the proper position of a spin dryer lid prior to activating robot-controlled loading of wafers into the spin dryer in order to prevent inadvertent collision of the lid with the robot.
A still further object of the present invention is to provide a dryer lid/robot collision prevention system which utilizes at least one photoelectric sensor to ascertain the position of a spin dryer lid prior to actuating a wafer transfer robot in the transfer of wafer cassettes containing wafers into the spin dryer.